Check for Downloads dir's existence and create it if it doesn't exist.

It's possible that Downloads directory (for whatever reason) is not
available on disk, and thus all sorts of bugs happen (Can't create
folder/files, can't query files inside the folder). This will make sure
if DownloadStorageProvider#queryRoot is called, then we
will create the folder.

DO NOT MERGE Deleting downloads for removed uids on downloadprovider start

After uninstalling an app, if the system was shutdown before the
download provider received the broadcast for UID_REMOVED, another app
installed later in the same uid might be able to gain access to the
files downloaded by this app. Removing any such hanging downloads
at the start up of the download provider should fix this issue.

Test: Manually tested by uninstalling an app and killing and restarting
the process android.process.media, to check that the downloaded files of
the uninstalled app were deleted.

After uninstalling an app, if the system was shutdown before the
download provider received the broadcast for UID_REMOVED, another app
installed later in the same uid might be able to gain access to the
files downloaded by this app. Removing any such hanging downloads
at the start up of the download provider should fix this issue.

Test: Manually tested by uninstalling an app and killing and restarting
the process android.process.media, to check that the downloaded files of
the uninstalled app were deleted.

DO NOT MERGE Deleting downloads for removed uids on downloadprovider start

After uninstalling an app, if the system was shutdown before the
download provider received the broadcast for UID_REMOVED, another app
installed later in the same uid might be able to gain access to the
files downloaded by this app. Removing any such hanging downloads
at the start up of the download provider should fix this issue.

Test: Manually tested by uninstalling an app and killing and restarting
the process android.process.media, to check that the downloaded files of
the uninstalled app were deleted.

After uninstalling an app, if the system was shutdown before the
download provider received the broadcast for UID_REMOVED, another app
installed later in the same uid might be able to gain access to the
files downloaded by this app. Removing any such hanging downloads
at the start up of the download provider should fix this issue.

Test: Manually tested by uninstalling an app and killing and restarting
the process android.process.media, to check that the downloaded files of
the uninstalled app were deleted.